1. Field of the Invention
The present invention relates generally to acoustic surface wave energy devices employing acoustical means for processing electrical input signals and yielding modified electrical output signals and, more particularly, concerns piezoelectric acoustic surface wave coupling devices in which the propagating energy is transferred from one acoustic wave propagating path or track at the surface of a first medium to a second acoustic wave propagating track at the surface of a second medium.
2. Description of the Prior Art
Prior art acoustic coupler and acoustic coupling filter devices having a pair of coupled propagation tracks have been described extensively in the literature, which includes descriptions of track changing directional coupler devices generally similar in nature to those of the L. P. Solie U.S. Pat. Nos. 3,898,592 for an "Acoustic Surface Wave Signal Processor", issued Aug. 5, 1975 and 4,079,342 for "Fanned Multistrip Coupler Filters", issued Mar. 14, 1978, both patents being assigned to Sperry Rand Corporation. Of interest also are the concepts described in the technical paper entitled: "Surface-Acoustic-Wave Multiplexing Techniques" by H. Van de Vaart and L. P. Solie, Proceedings of th I.E.E.E., May 1976, Vol. 4, No. 5, pages 688 to 691.
In more particular, there are several piezoelectric substrate materials known to cooperate in the generation and propagation of surface acoustic waves along such tracks, including LiNbO.sub.3, LiTaO.sub.3, quartz, and the like. It will be apparent that, if a coupler device is to have a relatively large band width, the selected surface wave medium for the coupler must have a high piezoelectric coupling factor, or else its insertion loss will be undesirably high. A second desirable attribute of the surface wave medium is that it have a characteristic delay that varies little with temperature over the operating temperature range of the apparatus in which the coupler is to be used.
The best known materials because of their ready availability in acceptable substrate size and of useful mechanical properties and the like are LiNbO.sub.3 and quartz. A commonly used material, LiNbO.sub.3, has a desirably high piezoelectric coupling, but it has a delay variation of 92 parts per million per degree Centigrade temperature variation, which is objectionably large. By contrast, quartz, particularly ST-cut quartz, as a second generally available material, has a fairly wide normally useful temperature range wherein there is substantially zero delay variation with temperature. However, the piezoelectric coupling constant of ST-cut quartz is distinctly lower than that of LiNbO.sub.3. Thus, ST-cut quartz devices can operate only over a relative band width of four percent with minimal insertion loss, while a LiNbO.sub.3 device can operate over a 25 percent relative band width with the same insertion losses.